As electronic components are getting smaller and smaller along with the internal structures in integrated circuits, it is getting easier to either completely destroy or otherwise impair electronic components. In particular, many integrated circuits are highly susceptible to damage from the discharge of static electricity. Electrostatic discharge (ESD) is the transfer of an electrostatic charge between bodies at different electrostatic potentials (voltages), caused by direct contact or induced by an electrostatic field. The discharge of static electricity, or ESD is a critical problem for the electronics industry.
Device failures that result from ESD events are not always immediately catastrophic or apparent. Often, the device is only slightly weakened but is less able to withstand normal operating stresses and, hence, may result in a reliability problem. Therefore, various ESD protection circuits must be included in the device to protect the various components.
When an ESD pulse occurs on a transistor, the extremely high voltage of the ESD pulse can break down the transistor and can potentially cause permanent damage. Consequently, the input/output pads of an integrated circuit need to be protected from ESD pulses so they are not damaged.
One of the issues faced with the design of ESD protection devices is the problem of localized high current in large parallel ESD devices. Ballast resistance is commonly used to reduce the amount of current that flows through a particular branch of an ESD device. By limiting the amount of ESD current that flows though a particular branch of an ESD device, the device as a whole will be able to handle more current before device failure. Ballasting provides a more uniform distribution of the current and therefore results in less current crowding and the failure current is increased. Such ballasting resistance is accomplished by creating separate resistance regions, or by using a silicide blocking mask to increase the drain resistance of the ESD device. Using separate ballasting resistors, however, increases the physical size of the ESD device which increases die cost, and blocking silicide requires an extra mask which increases the cost of a mask set. Silicide blocking also adds processing cost because of the extra processing step required. Thus there is a need for ESD protection structures with effective ballasting that are compact in area and avoid the cost of extra masks.